Polymeric films, like polyethylene and polypropylene film, are produced by extrusion. Each of such film products may contain a plurality of layers, typically three or more layers, of film.
In the art, the equipment used for producing the polymeric films consists of as many feeding lines as there are layers in the film. Each feeding line has a mixing and holding hopper in which the components of the master batch, including chemical, such as polymers, colorants and other additions, are mixed prior to directing the film to an electrically heated cylindrical container with a transportation screw. During the transfer of the chemicals, heat is applied to raise the temperature suited for molding the polymeric film. The friction of the product in the extruder generates the pressure needed to push the product through the extrusion die. Before leaving the extruder, the polymeric film product passes a filter to separate the coarse and unwanted materials. In the extrusion die, the product passes through various channels to obtain an equal distribution of the product over the complete length of the die.
When leaving the die, the polymeric film is cooled in order to crystallize the polymer, and to make it more difficult to damage when handling. In case of a linear die, the cooling can be done by blowing air on the film, and by passing the film over a water-cooled cylinder. Once the film is cooled, it is cut at a desired width and then coiled. In case of a circular die, the film is cooled by air blown outside through an annular distribution system, and inside the film via blow extrusion.
Once the product is heated, physical (e.g., thermal decomposition) and chemical (e.g., oxidation and carbonization) phenomenon can create hard, black particles which either remain on the filter in the extruder or deposit on the outlet of the die. In both cases, the production process has to be stopped because, in the event the filter is clogged, the product no longer enters the extrusion die at the desired flow or (in case of deposits on the die exit) the produced film is damaged (e.g., unequal thickness) by the deposits. At a minimum, the deposit adheres to the die outlet, and causes detrimental and undesired effects on the quality of the film, including the formation of scratches, uneven thickness of the film, and decreased deformation of the film. During the shutdown, the extrusion installation has to be cleaned. The entire operation results in a loss of product and productivity. With the frequent shutdowns of the equipment, it may also result in further difficulties for producing larger coils of films.
Various pieces of prior art have unsuccessfully attempted to solve the problems of producing polymeric films. The problem of particle buildup in the die of the extruders has been recognized. Various techniques such as die design changes and chemical additives has been proposed and applied commercially. A review of the studies and solutions related to the subject is provided. See, J. D. Gander et al., "Review of Die Lip Buildup in Plastic Extrusion", Polymer Engineering and Science, July 1997, Vol. 37, No. 7, pp. 1113-1126. The use of inert gas for complex machining at the level of spinner extruders in the production of polymer fibers unrelated to linear has been described in U.S. Pat. Nos. 3,847,522 and 2,252,689. The use of chemical solvents to clean the die lip was described in U.S. Pat. No. 5,391,071.
Further, more complex methods requiring heat to raise the temperature of nitrogen prior to the application of inert gas for installing in the extruder is found in U.S. Pat. No. 3,847,522.
It is, therefore, an object of this invention to provide for a method and system for producing polymeric film in which the extrusion die is not susceptible to damage or formation of undesired oxidized product which interferes with the polymer film extrusion process.
It is an object of this invention to provide for a method and system for inerting an atmosphere in selected area of open spaces during the production of polymeric film by extrusion.